This project will use extracellular electrophysiological recording to study the effects of background noise on the neural representation of pure tone stimuli. Neural responses will be measured in terms of average discharge rates for single units. Dynamic range properties of single-unit responses will be derived from measures of rate-level functions that are obtained in quiet and in the presence of continuous background noise. Dynamic range measures will be sampled in the dorsal cochlear nucleus, inferior colliculus and primary auditory cortex of behaving cats. Inhibitory mechanisms that shape the neural representation of tones in noise at each of these three levels of auditory processing will be described by frequency response maps, responses to bandpass noise, and responses to broad band noise with notched spectra. This research is based on independent observations that inhibitory influences increasingly reduce auditory responses to continuous noise at higher levels of the auditory system. A reduction in noise-driven activity potentially enhances the rate representation of signals that occur in the presence of background noise by decreasing the negative effects of noise adaptation The primary objectives of this proposal are to demonstrate changes in noise adaptation across three levels of auditory processing, to reveal neural inhibitory mechanisms that contribute to changes in noise-driven activity, and to measure the functional consequences of these changes in terms of the quality of neural representations for signals in noise. Although some of these objectives have been addressed in the nervous system of anesthetized and decerebrate cats, a comprehensive analysis of the neural encoding of pure tones in noisy environments has not been performed in behaving cats. Data obtained in behaving cats are critical to our understanding of noise adaptation because anesthesia substantially alters activity of descending efferent projections to the auditory periphery and inhibitory mechanisms at each subsequent level of auditory processing.